1. Field of the Invention
The present invention generally relates to electronic power converters and regulators and, more particularly, to switched power converters and reduction of electromagnetic interference noise generated therein.
2. Description of the Prior Art
Virtually all present-day electronic devices use direct current (DC) power initially derived from batteries or alternating current (AC) power distribution networks and, often, a combination thereof such as using rechargeable batteries which may be recharged using AC power. Therefore, power converters, many of which may have a voltage regulation function, are almost universally required to derive a DC voltage appropriate to the circuitry of the device and which, for present and foreseeable designs of microprocessors and the like may be a very low voltage (e.g. 0.8 volts or less) at very high currents (e.g. 150 amperes or more). In general, to improve efficiency, particularly for such voltages and currents, switched power converters and regulators are preferred to analog regulators across which a voltage drop must be established at high current; implying particularly high power dissipation from the regulator.
All switched power converters generate electromagnetic interference (EMI) which may be radiated and/or reflected into the power lines. Therefore the EMI generated must meet standards of the Federal Communications Commission and others.
EMI is composed principally of components referred to as common mode (CM) and differential mode (DM) noise. CM noise is difficult to control since it is caused by the parasitic capacitance between circuits of the power converter and ground. Typically, an EMI filter is used to suppress CM noise since the parasitics cannot be eliminated even though EMI filters are costly, consume significant volume and footprint and power; reducing efficiency of the power converter. Adequacy of EMI filters to suppress CM noise is also marginal, particularly at higher dv/dt on nodes of the power converter circuit resulting from operation at very low voltage output.
Since CM noise is caused by charging and discharging parasitic capacitance and high dv/dt is inevitable in switching power converters, it is desirable for the parasitic capacitances to be as small as possible. However, power devices included in power converters consume power and generate heat which must be dissipated through use of a heat sink. The parasitic capacitance between the heat sink (which may or may not be grounded) and the power devices is thus also inevitable; the noise current flowing from the power devices through the heat sink to ground. This parasitic capacitance is particularly difficult to reduce consistent with providing required heat dissipation.